Once the reward gets small and transaction fee income goes down as people get savvy, miners will disappear and the network will be in serious trouble.
As the BTC economy grows, the number of transactions and thus the amount collected in transaction fees will grow. Assuming BTC is still around and going strong, by several years from now I would expect the amount of transaction fees per block to be at or over the 50 BTC level. Just think if any significant portion of online transactions was done in BTC, you are talking about millions of transactions per day, and at even 1 million transactions per day and a .01 fee per transaction, you're talking around 70 btc in transaction fees per block on average.